1. Field of the Invention
This invention relates to an improvement in the process for the production of ethylene oxide through the silver catalyzed vapor phase oxidation of ethylene and, more particularly, relates to an economic process for producing ethylene oxide which utilizes suitable semipermeable membrane separation units, permits the usage of a cheap, low purity oxygen feedstock, and minimizes the unreacted ethylene losses by a selective removal of argon and CO.sub.2 diluents from the process recycle gas stream.
2. Description of the Prior Art
The production of ethylene oxide is one of the most important commercial reactions in the world, with a current annual production of about eight million tons/year worldwide. Frequently the operation of a process of this magnitude within the realm of commercial practicality can often depend upon the ability to increase, even by only relatively small amounts, the ethylene oxide yield or, conversely, the ability to reduce, even by only relatively small amounts, the costs of a variety of essential elements in the process.
In one major embodiment of the process, the oxygen supplied for reacting with ethylene is obtained either from an expensive oxygen source providing essential relatively pure oxygen, i.e., at least, 95+%, or, alternatively, a cheaper, less pure stream supplying primarily molecular oxygen, along with a lesser but significant fraction of one or more diluents, e.g., nitrogen, argon and the like. Such a second source of oxygen can be obtained, for example, from air treated by suitable separation processes (Pressure Swing Adsorption), typically creating a gas having an oxygen content of about 90 mole %, or less. When added to the process cycle gas, usually in conjunction with a makeup ethylene feed, the oxygen and ethylene become mixed with a controlled amount of other diluents, other reaction products and contaminants, as for example, carbon dioxide, argon, nitrogen, methane, etc. The precise number and concentration of each ingredient which can be included is a significant consideration in forming a desirable reaction gas composition, since, among other considerations, one must avoid a composition having very high and unstable concentrations of oxygen and/or ethylene. Otherwise, a total combustion of the ethylene content, as well as the danger of an explosion caused by exceeding the flammability limit of the gas composition, can take place. CO.sub.2 is a particularly important diluent, since it is the major byproduct of the vapor phase process forming ethylene oxide. Depending on the reactor feed composition, catalyst type, age, etc., the process results in the production of 0.4-1.0 moles of CO.sub.2 per mole of ethylene oxide. Thus, a method of selectively monitoring and economically removing excess CO.sub.2, and other excess amount of impurities such as argon, while avoiding a substantial loss in reactant gas, particularly ethylene, has been a long felt need in the art.
In large scale plants currently in use, excess CO.sub.2 is usually chemically absorbed from the reaction recycle gas by contacting it with a hot potassium carbonate solution, stripping it out using steam, and vented into the atmosphere with a minimal loss of ethylene. However, such absorption units are quite expensive, and also require substantial amounts of energy during utilization; consequently in smaller size commercial plants which are unable to afford such expensive units the art has been forced to use a less expensive, and much less selective CO.sub.2 purge system which vents a substantial amount of ethylene along with the excess CO.sub.2. It is only when the economies of scale produced by large plants can be obtained does the expensive carbonate chemical absorption system, its use saving substantial amounts of ethylene purge losses, become truly feasible.
U.S. Pat. No. 3,083,213 is an early teaching on the undesirability of the presence of argon, particularly regarding the flammability issue, in a disclosed ethylene oxide process. In commercial ethylene oxide systems the art has controlled the argon level in the recycle gas by a venting, or bleeding mechanism, which of course, also results in the loss of a substantial amount of ethylene, resulting in a reduced ethylene oxide yield. U.S. Pat. No. 3,119,837 discloses that methane can be a suitable diluent under certain circumstances in an ethylene oxide process. British Pat. No. 1,321,095 discloses an ethylene oxide process wherein ethylene levels as high as 40-80% by volume are permitted, and in which several diluents are controlled through means of a vent mechanism.
The usage of membranes to separate a wide variety of gaseous components has become increasingly important in recent years, although its usage is still believed unknown in the ethylene oxide art. In copending case Ser. No. 729,431, of Brian Ozero, filed concurrently with the present application, the usage of semipermeable membranes to selectively purge argon from a fraction of an ethylene oxide recycle gas stream is disclosed; the process utilizing a high purity oxygen feed stream and a hot carbonate system for the great majority of the CO.sub.2 removal.
It is, therefore, an object of this invention to provide a process for the selective one-step removal of CO.sub.2 and argon from the ethylene-oxygen recycle gas mixture.
It is another object of this invention to provide a process for the removal of CO.sub.2 and argon, along with the maintaining of a desired recycle gas composition, without an accompanying substantial loss of ethylene, in the ethylene-oxygen recycle gas.
It is still another object of this invention to provide a process for the economic, semipermeable membrane assisted removal of CO.sub.2 and argon from the recycle gas in conjunction with the usage of a low-purity oxygen feedstock, in an ethylene oxide process.